Magnesium is known as the lightest metal among metal materials used for structures, and has a specific gravity of 1.74 (density g/cm3, 20° C.). The magnesium can have a higher strength by adding a variety of elements and alloying them. Accordingly, the recently magnesium alloy can be used as housing for small portable machines such as cellular phones or mobile machines, housing for notebook computers, or components for automobiles, etc. Particularly, the magnesium alloy including a large amount of aluminum (for example, ASTM American Standard for Testing and Materials: AZ91) has a high corrosion resistance or a strength, and thus a great demand for the magnesium alloy is expected.
However, since the magnesium alloy has a hcp structure (hexagonal close-packed structure) which is poor in plastic processability, the magnesium alloy products used as the above-mentioned housing are mainly cast materials produced by a die casting or thixo molding method. As other magnesium alloys, for example, AZ31 that is relatively easily subjected to a plasticity process is used for housing by rolling an ingot-cast cast material for producing a plate and subsequently press-molding the plate (see Patent Literature 1, as an analogous technique).    Patent Literature 1: Japanese Patent Unexamined Publication JP-A-2005-2378